Risk assessment-based design method for deep complex formation wellbore structure

ABSTRACT

A risk assessment-based design method for a deep complex formation wellbore structure includes: (1) preliminarily determining casing layers and setting depths; (2) calculating to obtain the risk coefficients of each layer of casing; (3) analyzing and coordinating, according to the principle that a shallow casing shares more risks and a deep casing shares less risks, the risks of each layer of casing: determining whether the risk coefficients of each layer of casing are greater than a safety threshold value K; checking the setting depth: if the safety coefficient of an ith-layer casing satisfies R Ni &gt;K, selecting a casing layer with the minimum safety coefficient from upper casing layers, and deepening the setting depth h of the casing layer; and (4) repeating the steps (2) to (3) until the casing risk coefficients of each layer of casing are less than the safety threshold value K.

CROSS REFERENCES

This application is the continuation-in-part application of U.S. application Ser. No. 17/033,849 Filed on 27 Sep. 2020 which claims priority to Chinese Application No. CN2019111037553 filed on 13 Nov. 2019, the entire contents of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a risk assessment-based design method for a deep complex formation wellbore structure, and belongs to the technical field of oil and gas drilling.

BACKGROUND OF THE INVENTION

Wellbore structure design is one of the important contents of drilling engineering design, and the rationality of a wellbore structure design scheme directly affects safe and efficient implementation of drilling and completion construction.

There are many factors affecting the wellbore structure design, mainly including: safe density window of drilling fluid, a geological setting position, a geological target, drilling cost and the like. Through the research and development of domestic and foreign experts and scholars, basic methods for wellbore structure designs of bottom to top, top to bottom, middle to two sides and mixed designs are gradually formed, which provides a guarantee for safe and efficient construction of drilling and completion in different regions, reservoirs and working conditions. However, as oil and gas exploration gradually moves into the fields of deep water and deep formation, the complexity and uncertainty of a deep formation bring greater challenges to the wellbore structure design: for example, the prediction accuracy of formation pressure is an important guarantee for the rationality of the wellbore structure design, but the prediction of the formation pressure before drilling at present has the problems of high upper formation prediction accuracy and low deep formation prediction accuracy, which results in that, in the wellbore structure design and construction processes, a shallow formation has redundancy in wellbore structure safety, while the deep formation often has risks in wellbore structure safety due to large prediction error of the formation pressure before drilling, and downhole complex situations often occur because of an imperfect wellbore structure design in a construction process. On the other hand, the design coefficient of a common wellbore structure at present is a value range recommended according to a drilling design manual and regional characteristics. In design, only one fixed value can be selected within the value range for designing according to experience and regional drilling data. As a result, the design coefficient of the whole well is a single numerical value, and if the design coefficient is selected too large, there may be redundancy for the shallow formation; and if the design coefficient is selected too small, it may be insufficient for the deep formation.

Therefore, it is necessary to develop a wellbore structure design method with the coordination of risks of each layer of casing based on the concept of risk assessment aiming at the characteristics of deep complex formation drilling and considering the formation prediction error of different well depths and the risk bearing capacity of each layer of wellbore structure.

SUMMARY OF THE INVENTION

Aiming at the defects in the prior art, the present invention discloses a risk assessment-based design method for a deep complex formation wellbore structure.

SUMMARY OF THE INVENTION

According to the present invention, in view of the characteristics of insufficient understanding of deep formation information and frequent occurrence of downhole complex situations, the risk of a deep wellbore structure is moderately moved upwards by coordinating the risks borne by the casings of all layers, more design space is provided for the casing layers and the setting depth of a deep formation, the comprehensive risk of the whole wellbore structure is reduced to the maximum extent, and a guarantee for safe and efficient drilling is provided.

The specific technical scheme of the present invention is as follows:

A risk assessment-based design method for a deep complex formation wellbore structure, including:

-   -   a. preliminarily determining casing layers and setting depths;     -   b. calculating risk coefficients of each layer of casing;     -   c. analyzing and coordinating, according to a principle that a         shallow casing shares more risks and a deep casing shares less         risks, the risks of each layer of casing:

determining whether the risk coefficients of each layer of casing are greater than a safety threshold value K, and setting the safety threshold value K according to the safety requirement of a target well;

checking the setting depths: if the risk coefficient of an ith-layer casing satisfies R_(Ni)>K, selecting a casing layer with the minimum risk coefficient from upper casing layers, and deepening the setting depth h of the casing layer; and

repeating (2) to (3) until the risk coefficients of each layer of casing are less than the safety threshold value K, the safety threshold value K of ranges is from 0.4 to 0.5;

Step (4) is used to determine the casing running depth for the comprehensive risk of the whole wellbore structure is reduced to the maximum extent, and a guarantee for safe and efficient drilling is provided.

According to the present invention, preferably, the method for preliminarily determining the casing layers and the setting depths in (1) at least includes:

-   -   a. determining a geological setting position; namely,         determining a setting horizon according to geological data,         where the “determining a setting point” here is a necessary link         in the wellbore structure design, that is, a blocked horizon is         determined by analyzing the geological data and regional         drilling data according to the horizon and the depth at which         the geology is complex and a downhole accident occurs easily, so         that a layer of casing must be designed correspondingly for         blocking the setting position at the depth (horizon) in actual         construction;     -   b. preliminarily determining a safety pressure window, wherein         the safety pressure window is preliminarily determined according         to prediction results of formation pore pressure, formation         fracture pressure and formation collapse pressure before         drilling and a pressure balance relationship of an open hole         section; and     -   c. preliminarily determining the casing layers and the setting         depths thereof by a conventional “top to bottom” design method         according to the results of (1-1) and (1-2) and a regional         wellbore structure design coefficient. The present invention         mainly focuses on the prominent problem of the drilling risks of         the deep formation in a deep drilling process; therefore, a “top         to bottom” method is adopted, which makes each layer of casing         go to the deepest and maximizes a design window of the deep         formation.

According to the present invention, preferably, the method for calculating to obtain the risk coefficient of each layer of casing in (2) is as follows:

(2-1) Probabilistic Distribution of Formation Pressure

the prediction error ΔP_(i) of the formation pressure P_(i) is a function of the well depth H:

ΔP _(i) =f(H)∈[P _(i0) ,P _(i1)]  (1)

in formula (1), P_(i0) is the lower limit value of the error, P_(i1) is the upper limit value of the error, and i represents the type of the formation pressure;

the explanation corresponding to the Formula (1) is: a formation information uncertainty-based formation pressure prediction method, and reference is made to “formation pressure prediction method with uncertainties” recorded in “Guan Z C, Ke K, Lu B P. An approach to casing program design with formation pressure uncertainties[J]. Journal of China University of Petroleum (Edition of Natural Science), 2009”;

the characteristic that the prediction error of the formation pressure before drilling is increased along with the increase of the well depth is introduced into the method of the present invention, and the prediction error of the formation pressure is given by others before design and is subjected to probabilistic distribution in the present invention,

where the probabilistic distribution of the prediction error of the formation pressure satisfies the following rule:

$\begin{matrix} {{f\left( P_{t} \right)} = {\frac{1}{\sqrt{2\pi}\sigma_{p_{i}}}{\exp\left( {- \frac{\left( {P_{i} - \frac{P_{i\; 1} + P_{i\; 0}}{2}} \right)^{2}}{2\sigma_{p_{i}}^{2}}} \right)}}} & (2) \end{matrix}$

in formula (2), σ_(P) _(i) is the standard deviation of P_(i) and is selected according to the prediction accuracy, and the value range is (0, 1); and in the present invention, the risk coefficient is calculated by the cumulative probability of the formation pressure and the cumulative probability of other wellbore structure design coefficients, where σ_(P) _(i) determines the “width” of a probabilistic distribution function, namely, the width of the upper and lower limits of a prediction function. The wider the probabilistic distribution function is, the more likely a real value falls into a predicted interval, that is to say, the higher the prediction accuracy is, but a large prediction range is not conducive to design.

The abovementioned formula (2) directly performs probabilistic distribution on the function of formula (1), which can greatly simplify a calculation process and improve the applicability of the method. The selection of the standard deviation σ_(P) _(i) does not depend on human experience, but depends on the formation pressure prediction accuracy at different depths calculated according to formula (1), which represents fluctuation amplitude of a formation pressure prediction error, that is, the prediction accuracy. For a well section with a large prediction error, a large value is selected; and for the well section with a small prediction error, a small value is selected.

According to the present invention, a specific error does not need to be obtained, and the prediction accuracy of the function is controlled by selecting different values. For example:

for a shallow formation with high prediction accuracy of the formation pressure, in order to increase the design window of a wellbore structure, the width of the upper and lower limits of the prediction function can be reduced moderately, and σ_(P) _(i) is selected between 0.4 and 0.6;

and for the deep formation with low prediction accuracy of the formation pressure, in order to reduce the risk of the wellbore structure, the width of the upper and lower limits of the prediction function can be increased moderately, and σ_(P) _(i) is selected between 0.6 and 0.8.

The cumulative probability corresponding to the predicted value P_(i) of the formation pressure is:

$\begin{matrix} {{P\left( P_{i} \right)} = {\int_{- \infty}^{P_{i}}{\frac{1}{\sigma_{p_{i}}\sqrt{2\pi}}e^{- \frac{{({P_{i} - \frac{P_{i\; 1} + P_{i\; 0}}{2}})}^{2}}{2\sigma_{p_{i}}^{2}}}{dP}_{i}}}} & (3) \end{matrix}$

for the formation pore pressure, the prediction error is ΔP_(p)∈[P_(p0), P_(p2)], and for the formation fracture pressure, the prediction error is ΔP_(f)∈[P_(f0), P_(f1)];

(2-2) Probabilistic Distribution of Wellbore Structure Design Coefficient

if the value range of the wellbore structure design coefficient K is [K₀, K₁], then the probabilistic distribution formula thereof is as follows:

$\begin{matrix} {{f(K)} = {\frac{1}{\sqrt{2\pi}\sigma_{K}}{\exp\left( {- \frac{\left( {K - \frac{K_{1} + K_{0}}{2}} \right)^{2}}{2\sigma_{K}^{2}}} \right)}}} & (4) \end{matrix}$

in formula (4), σ_(K) is the standard deviation of K and is actually selected according to the drilling of a region where a target well is located, and the value range is (0, 1);

the wellbore structure design coefficient K includes: a kick tolerance, a fracture pressure safety factor, a suction pressure factor, an exciting pressure coefficient, an additional drilling fluid density, and the like; these coefficients are mainly set to make up for the potential risks caused by a formation pressure prediction error, a fluctuating pressure during drilling construction, and the like, so as to determine a safe drilling fluid density window. The wellbore structure design is performed in the determined safe drilling fluid density window. It can be seen that whether the selection of a design coefficient is reasonable or not has a great impact on the rationality and safety of wellbore structure design results.

At present, the wellbore structure design coefficient is an empirical value with regional characteristics, that is, it is selected within a recommended range, that is, K0 and K1 in formula (4). For example, in the Gulf of Mexico, the kick tolerance is generally 0.06 to 0.08, but in deep water in West Africa, the kick tolerance is generally 0.04 to 0.06. In design, engineering technicians generally select a fixed value within the abovementioned range according to the drilling experience in the region, which leads to certain inaccuracy and risk in the design results.

The present invention fully considers that the design coefficient has regional characteristics, and probabilistic distribution is performed on the value range of the wellbore structure design coefficient by using formula (4), so that an original empirical single value becomes a distribution form with probability information. In formula (4), K0 and K1 are respectively a minimum value and a maximum value of the recommended wellbore structure design coefficients in the region;

if the occurrence frequency of a downhole engineering risk in a regional drilling practice is low, a relatively small σ_(K) value can be selected with regard to a shallow wellbore structure design coefficient; if the occurrence frequency of the downhole engineering risk in the regional drilling practice is high, a relatively large σ_(K) value can be selected with regard to a deep wellbore structure design coefficient; for example: for the shallow formation, σ_(K) is selected between 0.4 and 0.6; for the deep formation, σ_(K) is selected between 0.6 and 0.8;

a credibility J is set to obtain the distribution interval of each design coefficient K as [f₀(K), f_(n)(K)]; in the distribution interval, the cumulative probability corresponding to the design coefficient f_(i)(K) is:

$\begin{matrix} {{P\left( {f_{i}(K)} \right)} = {\int_{- \infty}^{f_{i}{(K)}}{\frac{1}{\sigma_{K}\sqrt{2\pi}}e^{\frac{{({{f_{i}{(K)}}\frac{{f_{1}{(K)}} + {f_{0}{(K)}}}{2}})}^{2}}{2\sigma_{\kappa}^{2}}}{d\left( {f_{i}(K)} \right)}}}} & (5) \end{matrix}$

the distribution intervals of kick tolerance S_(k), formation fracture pressure safety factor S_(f), additional drilling fluid density Δρ and suction pressure factor S_(b) are respectively expressed as: [f₀(S_(k)), f_(n)(S_(k))], [f₀(S_(f)), f_(n)(S_(f))], [f₀(Δρ), f_(n)(Δφ] and [f₀(S_(b)), f_(n)(S_(b))];

The calculation of the cumulative probability is to obtain the specific probability corresponding to a certain wellbore structure design coefficient value.

By means of the probabilistic distribution of the wellbore structure design coefficient, that is, solving an integral by formula 4, the specific probability that the wellbore structure design coefficient value is greater than a certain value, that is, the cumulative probability can be obtained. Thus, the corresponding specific probability when the wellbore structure design coefficient is any value within the value range can be obtained.

According to the present invention, preferably, the value of the credibility J is 70%-95%;

at present, a common wellbore structure design coefficient is a value range recommended according to a drilling design manual and regional characteristics, and a fixed value is selected from the value range for designing; according to the present invention, a probability statistical method is adopted, regional well structure design coefficients are subjected to probabilistic distribution, and different design coefficients are selected with regard to the risk bearing capacity of each casing layer;

(2-3) Downhole Engineering Risk Calculation for an Nth-Layer Casing at the Well Depth H

the downhole engineering risk R(H) at the well depth H is calculated according to the pressure balance relationship:

$\begin{matrix} {\mspace{79mu}{{{kick}\mspace{14mu}{risk}\text{:}}{{R_{JY}(H)} = {{m\left\lbrack {1 - {P\left( {P_{p}(H)} \right)}} \right\rbrack} \times \left\lbrack {1 - {P\left( {f_{n}\left( S_{b} \right)} \right)}} \right\rbrack \times \left\lbrack {1 - {P\left( {f_{n}\left( {\Delta\;\rho} \right)} \right)}} \right\rbrack}}\mspace{20mu}{{where},{m = \left\{ \begin{matrix} 0 & {\rho_{m} > {{P_{p}(H)} + {f_{n}\left( S_{b} \right)} + {f_{n}\left( {\Delta\;\rho} \right)}}} \\ 1 & {\rho_{m} \leq {{P_{p}(H)} + {f_{n}\left( S_{b} \right)} + {f_{n}\left( {\Delta\;\rho} \right)}}} \end{matrix} \right.}}}} & (6) \end{matrix}$

risk of lost circulation:

$\begin{matrix} {{{R_{JL}(H)} = {m \times {P\left( {P_{f\; 0}(H)} \right)} \times \left\lbrack {1 - {P\left( {f_{n}\left( S_{k} \right)} \right)}} \right\rbrack \times \left\lbrack {1 - {P\left( {f_{n}\left( S_{f} \right)} \right)}} \right\rbrack}}{{where},{m = \left\{ \begin{matrix} 0 & {\rho_{m} < {{{f_{n}\left( S_{k} \right)} \times \frac{H}{H_{n - 1}}} + {f_{n}\left( S_{f} \right)} + {P_{f0}(H)}}} \\ 1 & {\rho_{m} \geq {{{f_{n}\left( S_{k} \right)} \times \frac{H}{H_{n - 1}}} + {f_{n}\left( S_{f} \right)} + {P_{f0}(H)}}} \end{matrix} \right.}}} & (7) \end{matrix}$

in formulas (6) and (7), ρ_(m) is the equivalent density of drilling fluid, and H_(n-1) is the depth of the last casing shoe;

in the technical field, the downhole risk can be determined according to a pressure balance relationship in the wellbore. For example, when the wellbore pressure is lower than the formation pore pressure, well kick will occur; when the wellbore pressure is higher than the formation fracture pressure, lost circulation will occur. However, according to the abovementioned theory, it can only judge that whether relevant risks occur or not, and the accuracy of judgment results depends heavily on the prediction accuracy of the formation pressure and the rationality of the design coefficient.

Combined with formula (6), the specific probability of the occurrence of the well kick risk is obtained by calculating the probability that the wellbore pressure is less than the sum of the formation pore pressure and the relevant wellbore structure design coefficient on the basis of probabilisticizing the formation pressure prediction error and the wellbore structure design coefficient.

Combined with formula (7), the specific probability of the occurrence of the risk of lost circulation is obtained by calculating the probability that the wellbore pressure is greater than the sum of the formation fracture pressure and the relevant wellbore structure design coefficient on the basis of probabilisticizing the formation pressure prediction error and the wellbore structure design coefficient;

(2-4) Determination of Risk Coefficients of Each Layer of Casing

the downhole engineering risks at the well depth H calculated in (2-3) are integrated within the range of the layer of casing to obtain the overall risk coefficient R_(N) of the Nth-layer casing

R _(N)=∫_(H) _(n) ^(H) ^(m) (R _(JY)(H)+R _(JL)(H))dH  (8)

in formula (8), H_(n) is the minimum depth of the Nth-layer casing; and H_(m) is the maximum depth of the Nth-layer casing;

It is well known to those skilled in the art that: the uncertainty of deep formation information is strong and the prediction error of the formation pressure is large, so the drilling risk of deep formation is greater than that of shallow formation. The running depth of casings at all layers of the wellbore structure has a significant impact on the drilling risk. If those skilled in the art encounter the drilling risk of the deep formation during drilling, the relevant risks are usually avoided by adjusting the running depth of the casings. However, this method is relatively passive, and remedial measures are usually taken after downhole risks occur. At present, in the wellbore structure design, the potential risks at all layers are not taken into full account in the optimization design before drilling. The main technical difficult point is that there is no scientific means to quantitatively evaluate the risk value of the casings at all layers before drilling.

In formula (8), specific risks of the casings at all layers are defined, so as to solve the specific risks of the casings at all layers under a certain wellbore structure solution, thereby providing scientific data support for overall planning and quantitative optimization of running depth of the casings at all layers before drilling.

The present invention has the technical advantages that:

According to the present invention, the above defects can be overcome by performing probabilistic distribution on each of the design coefficients and the prediction errors of the formation pressure and selecting the formation pressure prediction values and the design coefficients of different accuracy for different depths. Meanwhile, the risk coefficient of each layer of casing further can be calculated on that basis, the risks borne by each layer of casing are coordinated, and the overall wellbore structure risk is comprehensively reduced, so that the present invention has great advantages for the wellbore structure design of a deep well complex formation. According to the present invention, the wellbore structure design scheme that each layer of casing shares the risks based on the principle that the shallow casing shares more risks and the deep casing shares less risks is realized, which greatly reduces the safety risk caused by the wellbore structure during the drilling process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a specific design contrast diagram for a wellbore structure in an embodiment of the present invention.

FIG. 2 is a flowchart of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A specific implementation mode is introduced by taking a well A as an example. The design well depth is 6500 m; the kick tolerance S_(k)=0.05 g/cm³, the formation fracture pressure safety factor S_(t)=0.04 g/cm³, the additional drilling fluid density Δρ=0.05 g/cm³ and the suction pressure coefficient S_(b)=0.04 g/cm³. A formation pressure profile is as shown in FIG. 1.

A wellbore structure scheme of the well is preliminarily determined by adopting a top to bottom method according to (1) to (3) of the present invention.

In (2), the error cumulative probability formulas of the formation pore pressure and the formation fracture pressure are respectively obtained as follows by selecting the standard deviation of the formation pressure prediction error σ_(P) _(i) =0.6:

formation pore pressure:

${P\left( P_{P\; i} \right)} = {{\int_{- \infty}^{P_{pi}}{\frac{1}{\sigma\sqrt{2\pi}}e^{- \frac{{({P_{pi} - \frac{P_{{pi}\; 1} + P_{{pi}\; 0}}{2}})}^{2}}{2\sigma^{2}}}{dP}_{pi}}} = {\int_{- \infty}^{P_{pi}}{\frac{5}{3\sqrt{2\pi}}e^{- \frac{25{({P_{pi} - \frac{P_{{pi}\; 1} + P_{{pi}\; 0}}{2}})}^{2}}{18}}{dP}_{pi}}}}$

formation fracture pressure:

${P\left( P_{f\; i} \right)} = {{\int_{- \infty}^{P_{fi}}{\frac{1}{\sigma\sqrt{2\pi}}e^{- \frac{{({P_{fi} - \frac{P_{{fi}\; 1} + P_{{fi}\; 0}}{2}})}^{2}}{2\sigma^{2}}}{dP}_{fi}}} = {\int_{- \infty}^{P_{fi}}{\frac{5}{3\sqrt{2\pi}}e^{- \frac{25{({P_{fi} - \frac{P_{{fi}\; 1} + P_{{fi}\; 0}}{2}})}^{2}}{18}}{dP}_{fi}}}}$

According to the drilling experience of an adjoining well in the region, kick and lost circulation easily occur in the downhole with the depth interval of 4000 to 5000 m, so that the wellbore structure design coefficient σ_(P) _(i) =0.7 is selected for the depth greater than 4000 m, and σ_(P) _(i) =0.5 is selected for other depths. The credibility J=90% is set to obtain the distribution intervals and the cumulative probability calculation formulas of each of the coefficients as follows:

Kick tolerance: the distribution interval is

$\quad\left\{ \begin{matrix} \left\lbrack {0.04,0.06} \right\rbrack & {H < {4000\mspace{11mu} m}} \\ \left\lbrack {0.036,0.064} \right\rbrack & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

the cumulative probability formula is

${P\left( {f_{i}\left( S_{K} \right)} \right)} = \left\{ \begin{matrix} {\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\ {\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{2{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{49}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

Formation fracture pressure safety factor: the distribution interval is

$\quad\left\{ \begin{matrix} \left\lbrack {0.032,0.048} \right\rbrack & {H < {4000\mspace{11mu} m}} \\ \left\lbrack {0.03,0.05} \right\rbrack & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

the cumulative probability formula is

${P\left( {f_{i}\left( S_{f} \right)} \right)} = \left\{ \begin{matrix} {\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\ {\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{2{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{49}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

Additional drilling fluid density: the distribution interval is

$\quad\left\{ \begin{matrix} \left\lbrack {0.04,0.06} \right\rbrack & {H < {4000\mspace{11mu} m}} \\ \left\lbrack {0.03{6,}0.064} \right\rbrack & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

the cumulative probability formula is

${P\left( {f_{i}\left( S_{K} \right)} \right)} = \left\{ \begin{matrix} {\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\ {\int_{- \infty}^{f_{i}{(S_{K})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{2{({{f_{i}{(S_{K})}} + 0.05})}^{2}}{49}}{d\left( {f_{i}\left( S_{K} \right)} \right)}}} & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

Suction pressure coefficient: the distribution interval is

$\quad\left\{ \begin{matrix} \left\lbrack {0.032,0.048} \right\rbrack & {H < {4000\mspace{14mu} m}} \\ \left\lbrack {0.03,0.05} \right\rbrack & {H \geq {4000\mspace{14mu} m}} \end{matrix} \right.$

The cumulative probability formula is

${P\left( {f_{i}\left( S_{f} \right)} \right)} = \left\{ \begin{matrix} {\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{2}{\pi}}e^{{- 2}{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H < {4000\mspace{14mu} m}} \\ {\int_{- \infty}^{f_{i}{(S_{f})}}{\sqrt{\frac{50}{49\pi}}e^{- \frac{2{({{f_{i}{(S_{f})}} + 0.04})}^{2}}{49}}{d\left( {f_{i}\left( S_{f} \right)} \right)}}} & {H \geq {40000\mspace{14mu} m}} \end{matrix} \right.$

According to (2) to (3) in the present invention, in the embodiment, there are five layers of casings in total, and the downhole engineering risks of each layer of casing at different well depths are respectively calculated:

a first-layer casing: the kick risk R_(JY)=0; the lost circulation risk R_(JL)=0;

a second-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix} 0 & {H < {2000\mspace{14mu} m}} \\ {{9 \times 10^{6}x^{2}} - {{0.0}35x} + {3{5.2}79}} & {H \geq {2000\mspace{14mu} m}} \end{matrix};} \right.$

the lost circulation risk R_(JL)=0;

a third-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix} 0 & {H < {3500\mspace{14mu} m}} \\ {{2 \times 10^{6}x^{2}} - {{0.0}127x} - {2{3.6}57}} & {H \geq {3500\mspace{14mu} m}} \end{matrix};} \right.$

the lost circulation risk

$R_{JL} = \left\{ {\begin{matrix} {{2 \times 10^{6}x^{2}} - {0.0091x} + 10.052} & {H < {2150\mspace{14mu} m}} \\ 0 & {H \geq {2150\mspace{14mu} m}} \end{matrix};} \right.$

a fourth-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix} 0 & {H < {5150\mspace{14mu} m}} \\ {{2 \times 10^{6}x^{2}} - {{0.0}217x} + {5{6.0}27}} & {H \geq {5150\mspace{14mu} m}} \end{matrix};} \right.$

the lost circulation risk

$R_{JL} = \left\{ {\begin{matrix} {{{- 9} \times 10^{- 7}x^{2}} + {0.0067x} - 11.819} & {H < {3700\mspace{14mu} m}} \\ 0 & {H \geq {3700\mspace{14mu} m}} \end{matrix};} \right.$

a fifth-layer casing: the kick risk

$R_{JY} = \left\{ {\begin{matrix} 0 & {H < {6380\mspace{14mu} m}} \\ {{2 \times 10^{6}x^{2}} - {{0.0}255x} + {8{1.7}18}} & {H \geq {6380\mspace{14mu} m}} \end{matrix};} \right.$

the lost circulation risk

${R_{JL} = \left\{ \begin{matrix} {{{- 5} \times 10^{7}x^{2}} + {{0.0}055x} - {1{5.3}12}} & {H < {5400\mspace{14mu} m}} \\ 0 & {H \geq {5440\mspace{14mu} m}} \end{matrix} \right.}.$

According to (2) to (4) in the present invention, the overall risk coefficient of each layer of casing is obtained:

R ₁=0; R ₂=∫₃₀₀ ²¹⁰⁰(R _(JY)(H)+R _(JL)(H))dH=0.532;

R ₃=∫₂₁₀₀ ³⁶⁰⁰(R _(JY)(H)+R _(JL)(H))dH=0.483; R ₄=∫₃₆₀₀ ⁵¹⁰⁰(R _(JY)(H)+R _(JL)(H))dH=0.447;

R ₂=∫₅₁₀₀ ⁶⁵⁰⁰(R _(JY)(H)+R _(JL)(H))dH=0.408.

According to (3) to (4) in the present invention:

i): a safety threshold value K=0.5 is set according to actual conditions, wherein the overall risk coefficient of the second-layer casing is greater than the value;

ii): the setting depth of the first-layer casing is increased by 50 m;

iii): if the safety coefficient of the ith-layer casing satisfies R_(Ni)>K, the casing layer with the minimum safety coefficient is selected from upper casing layers, and the setting depth h of the casing layer is deepened; and

iv): until the risk coefficient of each layer of casing is less than the safety threshold value K, the safety threshold value K of ranges is from 0.4 to 0.5, the preferred value K is 0.5;

v): Step iv) is used to determine the casing running depth for the comprehensive risk of the whole wellbore structure is reduced to the maximum extent, and a guarantee for safe and efficient drilling is provided.

In order to reflect the technical advantages of the present invention, a comparison is made between embodiments of the present invention and comparative examples, where the comparative example described in Table 1 refers to a comparative technical scheme formed according to (1) to (2) of the present invention.

TABLE 1 Comparative Example The Present Embodiment Drilling Drilling Casing Fluid Casing Fluid Casing Setting Density Risk Setting Density Risk Layer Depth (g/cm³) Factor Depth (g/cm³) Factor 1 300 m 1.17 0 350 m 1.18 0.035 2 2100 m 1.35 0.532 2125 m 1.37 0.0486 3 3600 m 1.68 0.483 3635 m 1.72 0.043 4 5100 m 2.07 0.447 5110 m 2.23 0.0421 5 6500 m 2.63 0.408 6500 m 2.62 0.0413

In combination with Table 1 and FIG. 1, it can be seen that, after the processing and design of the method disclosed by the present invention, the risks of the five layers of casings are all less than the safety threshold value K=0.5, the casing setting depth of the shallow formation is deeper, and the depth of the open hole section of the deep formation (the setting depths of the fourth and fifth layers of casings) is reduced, which facilitates the reduction of downhole risk of the deep formation drilling, transfers the risk of a deep casing layer to a shallow casing layer, and reduces the overall risk. 

What is claimed:
 1. A risk assessment-based drilling method with a deep complex formation wellbore structure, wherein the deep complex formation wellbore structure is controlled by a device that comprises a computer-readable device and an instruction, and the device executes a processor for performing the following steps: (i) generating, by the processor, casing layers and a setting depth; (ii) creating a probabilistic distribution of formation pressure and prediction accuracy, by the processor, according to a prediction error of formation pressure and a well depth wherein the prediction error of formation pressure is correlated to the well depth;  wherein a probabilistic distribution of the prediction error of formation pressure is created, by the processor, based on a standard deviation and the prediction accuracy;  creating, by the processor, a cumulative probability corresponding to the formation pressure by integrating the probabilistic distribution of the prediction error of formation pressure wherein the cumulative probability is greater than the prediction error of formation pressure; creating, by the processor, a probabilistic distribution of wellbore structure design coefficient based on a threshold value of a safety coefficient and a standard deviation of the threshold value of the safety coefficient;  generating, by the processor, a distribution interval of the threshold value of the safety coefficient with a credibility and the cumulative probability; wherein the probabilistic distribution of wellbore structure design coefficient is greater than the cumulative probability, any specific value of the cumulative probability is within the range of the probabilistic distribution of wellbore structure design coefficient;  creating, by the processor, a downhole engineering risk at a specific well depth according to a pressure balance relationship which is created by the processor with a kick risk; the kick risk is resulted from a risk of lost circulation, an equivalent density of drilling fluid, a formation pore pressure, a minimum limit of the formation pore pressure, and the depth of a last casing shoe; wherein a wellbore pressure is less than the sum of the formation pore pressure and the probabilistic distribution of wellbore structure design coefficient; creating, by processor, a risk coefficient by integrating the downhole engineering risk, wherein a specific downhole engineering risk is within a specific well structure scheme; (iii) selecting, by the processor, a specific risk coefficient of each layer of casing; wherein the specific risk coefficient of each layer of casing is greater than the threshold value of the safety coefficient; selecting, by the processor, a specific setting depth wherein the safety coefficient is greater than the threshold value of the safety coefficient, selecting a casing layer with a minimum safety coefficient from the casing layers, and deepening the setting depth of the casing layer; (iv) repeating (ii) to (iii) until the risk coefficient of each layer of casing are less than the threshold value of the safety coefficient; and (v) positioning the deep complex formation wellbore structure for drilling based on the risk coefficient which provides data for overall planning and quantitative optimization of casing running depth at each level before drilling.
 2. The risk assessment-based drilling method according to claim 1, wherein the method in the step (i) further comprises: (a) measuring a geological setting position; (b) creating, by the processor, a safety pressure window based on prediction results of formation pore pressure, formation fracture pressure and formation collapse pressure before drilling and a pressure balance relationship of an open hole section; and (c) generating, by the processor, the casing layers and the setting depth based on the geological setting position, the safety pressure window and a regional wellbore structure design coefficient. 